docs: add en-US changelog

docs/markdown/changelog-v3.en-US.md

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+# Changelog
+
+### Tips
+
+The current document is the changelog of Vant 3.x. If you want to view the changelog of Vant 2.x, please visit [Vant 2.x Changelog](https://youzan.github.io/vant/#/en-US/changelog).
+
+### Intro
+
+Vant follows [Semantic Versioning 2.0.0](https://semver.org/lang/zh-CN/).
+
+**Release Schedule**
+
+- Patch version：released weekly, including features and bug fixes.
+- Minor version：released every one to two months, including backwards compatible features.
+- Major version：including breaking changes and new features.
+
+## Details
+
+### [v3.0.0-beta.6](https://github.com/youzan/vant/compare/v2.10.12...v3.0.0-beta.6)
+
+`2020-11-01`
+
+**Bug Fixes**
+
+- Calendar: watch maxData/minDate and reset [#7412](https://github.com/youzan/vant/issues/7412)
+- Swipe: incorrect lazy render when loop is false [#7465](https://github.com/youzan/vant/issues/7465)
+- Swipe: item should only rendered once [#7466](https://github.com/youzan/vant/issues/7466)
+- Tabs: skip initial animation [49e877](https://github.com/youzan/vant/commit/49e87756c70b33e1a56620ebee3c0aa53fb9fc86)
+- ActionBar: fix typing [#7440](https://github.com/youzan/vant/issues/7440) [#7442](https://github.com/youzan/vant/issues/7442)